JavaScript Interview Questions

Here is a list of common JavaScript interview questions with detailed answers to help you prepare for the interview as a JavaScript developer.

JavaScript continues to be a cornerstone of web development, powering dynamic and interactive experiences across the web.

As the language evolves, so does the complexity and scope of interview questions for JavaScript developers. Whether you’re a fresher developer preparing for your first job interview or a seasoned professional looking to brush up on the latest features, understanding the key concepts of JavaScript is crucial.

This list of JavaScript interview questions and answers aims to assist you with brushing up on JavaScript programming concepts to prepare for your interview. We have covered a range of topics, from fundamental principles to advanced features introduced in ES6 and beyond.

Through concise explanations and examples, we’ll explore essential interview questions that reflect the current trends and best practices in JavaScript development. Get ready to dive deep into the world of JavaScript and sharpen your skills for your next job interview.

JavaScript Questions and Answers

Q1. What is JavaScript, and can you explain its main features?

JavaScript is a high-level, interpreted scripting language used to make web pages interactive and build web applications. It can run in a web browser without needing to be compiled on the client side, enabling dynamic content updates and interactive features. Key features of JavaScript include:

1. Event-driven programming: JavaScript responds to user actions like clicks, form inputs, and page navigation, allowing for interactive web experiences.
2. Prototypal inheritance: Objects can inherit properties from other objects, enabling more flexible and less hierarchical inheritance structures than traditional class-based inheritance.
3. First-class functions: Functions in JavaScript are treated as first-class citizens, meaning they can be assigned to variables, passed as arguments, and returned from other functions, enabling higher-order functions.
4. Asynchronous programming: With features like callbacks, promises, and async/await, JavaScript can perform non-blocking operations, improving the performance of web applications by allowing tasks like data fetching to run in the background. These features allow asynchronous programming by JavaScript.
5. Cross-platform compatibility: JavaScript runs on any browser, providing a universal language for web development across different operating systems.

Q2. What are the differences between var, let, and const in JavaScript?

In JavaScript, “var”, “let“, and “const” are all used to declare variables, but they differ in terms of scope, hoisting, and reassignment.

1. var: Declares a variable with function scope or globally if declared outside of a function. It is hoisted, allowing it to be used before it’s declared in the code. Variables declared with “var” can be updated and re-declared within its scope.
2. let: Introduces block scope “({…})” for variables, which is more predictable and commonly used in modern JavaScript. “let” variables can be updated but not re-declared within the same scope. Unlike “var,” “let” is not hoisted, meaning it cannot be accessed before the declaration.
3. const: Also has block scope and is not hoisted. “const” is used to declare variables meant to be constants or not re-assigned after their initial assignment. While a “const” object itself cannot be re-assigned, the properties of that object can be updated.

In summary, the choice between “var”, “let“, and “const” depends on the variable’s intended mutability and the scope we want to work within, with “let” and”const” offering more predictability and safer code practices in block-scoped situations.

Q3. Explain event bubbling and event capturing in JavaScript.

Event bubbling and event capturing are two phases of event propagation in the Document Object Model (DOM) when an event occurs in an element within a nested structure.

• Event bubbling is the default behavior where an event starts from the target element that triggered the event and then bubbles up to the ancestor elements in the hierarchy. For example, if we click on a button inside a form, the click event will first be registered on the button and then propagate upwards to the form and so on, potentially up to the document object. This allows for a single handler on a parent element to listen for events that occur on its child elements.
• Event capturing (or event trickling down) occurs in the opposite direction. The event starts from the topmost parent element and trickles down to the target element. This phase precedes the bubbling phase, but it’s less commonly used. It allows for an event to be captured before it reaches its intended target.

Developers can specify the phase they want to listen for by using the third parameter of “addEventListener:” setting it to “true” enables capturing phase, and “false” (or omitting it, since “false” is the default value) uses the bubbling phase.

Q4. Can you briefly explain what is a Document Object Model (DOM)?

The Document Object Model (DOM) is a programming interface for web documents. It represents the page so that programs can change the document structure, style, and content. The DOM represents the document as a hierarchical tree of nodes, where each node represents a part of the document (for example, an element, attribute, or text node).

JavaScript interacts with the DOM to manipulate web pages dynamically. It can read and change the DOM, allowing for interactive content. For example, JavaScript can:

• Modify HTML content by changing element text or attributes.
• React to user events like clicks, keyboard input, or mouse movements by attaching event listeners to DOM elements.
• Create or remove elements to add or take away content dynamically.
• Change styles to alter the appearance of elements.

Through the DOM API, JavaScript provides a way to programmatically interact with HTML and CSS, enabling dynamic content updates, interactive forms, animations, and everything else that makes web pages feel ‘alive.’

Q5. What is a closure in JavaScript, and how does it work?

A closure in JavaScript is a feature where an inner function has access to the outer (enclosing) function’s variables and scope chain, even after the outer function has returned. This happens because closures allow a function to retain access to its lexical scope; that is, where it was declared, not where it is executed.

Closures work by the JavaScript engine, preserving the scope of an outer function in memory when an inner function is returned.

The inner function retains access to the variables, parameters, and functions of its outer function, allowing for powerful programming patterns like data encapsulation, currying, and factories.

Essentially, closures provide a way to maintain state in a functional programming language, enabling functions to have ‘private’ variables and methods.

Q6. Can you describe the prototype inheritance model in JavaScript?

In JavaScript, the prototype inheritance model is a mechanism that allows objects to inherit features from other objects.

Each JavaScript object has a property called “prototype”, which is a reference to another object.

When we attempt to access a property or method of an object, and it is not found on the object itself, the JavaScript engine looks up the property on the object’s prototype (its parent). However, if not found there, it continues with the prototype’s prototype, and so on, until it reaches the end of the prototype chain, which is typically the “Object.prototype”.

This model is different from classical inheritance used in languages like Java or C#, where classes inherit from other classes.

In JavaScript, inheritance is achieved through objects, not classes, allowing for more flexible and dynamic inheritance structures. This mechanism enables object composition and the sharing of functionality across different objects without needing a rigid class hierarchy.

Basically, prototype inheritance allows objects in JavaScript to inherit properties and methods from their prototypes, forming a chain that the engine consults in a search for properties and methods.

Q7. What is the ‘this’ keyword in JavaScript, and how does its value change?

The ‘this’ keyword in JavaScript is a context-sensitive reference that points to the object that is currently executing or invoking the function where ‘this’ is used. Its value can change based on how the function is called, and it has four main rules to determine its value:

1. Global Context: In the global execution context (outside of any function), ‘this’ refers to the global object, which is “window” in web browsers or “global” in Node.js.
2. Function Call: When a function is called as a regular function (e.g., “myFunction()”), ‘this’ refers to the global object in non-strict mode and “undefined” in strict mode.
3. Method Call: When a method is called as a property of an object (e.g., “myObject.myMethod()”), ‘this’ refers to the object the method is called on.
4. Constructor Call: When a function is used as a constructor with the new keyword (e.g., “new MyConstructor()”), ‘this’ refers to the newly created object instance.

Additionally, ‘this’ can be explicitly set using methods like “call()”, “apply()”, and “bind()”, allowing more control over its value in different contexts.

Q8. How do you handle asynchronous operations in JavaScript?

In JavaScript, asynchronous operations can be handled using several patterns, each providing a way to deal with operations like API calls, file reads, or timers without blocking the execution thread. The three main patterns are:

1. Callbacks: The oldest method, where a function is passed as an argument to an asynchronous function and is called when the operation completes. While straightforward, excessive use can lead to “callback hell,” where nested callbacks become difficult to read and maintain.
2. Promises: Introduced to address callback hell, a Promise is an object representing the eventual completion or failure of an asynchronous operation. Promises can be chained with “.then()” for success and “.catch()” for errors, allowing for more readable and manageable code.
3. Async/Await: The most modern syntax, async/await, makes asynchronous code look and behave a bit more like synchronous code. An “async” function returns a Promise, and “await” is used to pause the function’s execution until the Promise is settled. This syntax provides a cleaner and more intuitive way to handle asynchronous operations.

In practice, choosing between these methods depends on the specific requirements of the operation and personal or team coding preferences. However, async/await, being the most readable and easy to understand, is generally preferred for new projects.

Q9. What are JavaScript “Promises,” and how do they work?

JavaScript Promises are objects representing the eventual completion or failure of an asynchronous operation. They provide a cleaner, more manageable way to handle asynchronous tasks compared to callbacks. A Promise can be in one of three states:

1. Pending: The initial state where the operation has not been completed yet.
2. Fulfilled: Indicates that the operation was completed successfully.
3. Rejected: Indicates that the operation failed.

Promises work by attaching callbacks to them for handling the success (“then”) or failure (“catch”) of the asynchronous operation. The “then” method can be used to chain multiple promises, allowing for complex asynchronous flows to be handled more readable and linearly. The “catch” method is used to handle any errors that occur in the chain.

Q10. How do you create a “Promise” in JavaScript?

To create a Promise, we use the “new Promise” constructor and pass it an executor function that takes two arguments: “resolve” and “reject”. The executor function performs the asynchronous operation and calls “resolve” upon success, passing the result, or “reject” upon failure, passing the error.

Q11. Can you explain the concept of hoisting in JavaScript?

Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their containing scope before code execution begins. This means that variables and functions can be referenced before they are physically declared in the code. However, it’s important to distinguish between the hoisting of variables declared with “var”, “let”, “const”, and function declarations.

• Variables declared with “var” are hoisted and initialized with “undefined”, allowing them to be accessed in their containing scope before they are declared.
• Variables declared with “let” and “const” are also hoisted, but they are not initialized, leading to a “ReferenceError” if accessed before their declaration, a state known as the Temporal Dead Zone.
• Function declarations are hoisted and fully initialized, meaning the function can be called before its declaration in the code.

Hoisting allows for more flexibility in how functions and variables are organized, but it also requires careful attention to avoid errors related to the use before declaration, especially with “let” and “const.”

Q12. What is event delegation in JavaScript, and why is it useful?

Event delegation in JavaScript is a technique for handling events efficiently, especially in cases where there are multiple similar elements that require the same event handler.

Instead of adding an event listener to each individual element, event delegation involves adding a single event listener to a parent element. This listener then manages all events that bubble up from its child elements due to the event bubbling mechanism in the DOM.

This technique is useful for several reasons:

1. Performance: It reduces the number of event listeners needed in the application, which can improve performance and resource management, especially in complex applications with many elements.
2. Memory consumption: By reducing the number of event listeners, it also reduces the overall memory consumption of the application.
3. Dynamic elements: It simplifies event handling for dynamically added elements (elements added to the DOM after the initial page load) because the event listener is on the parent element, not the elements themselves.

Q13. How does the JavaScript event loop work?

The JavaScript event loop is a mechanism that allows JavaScript to perform long-running, non-blocking asynchronous operations despite being single-threaded.

It works by using a loop to manage and execute multiple tasks in a sequence, ensuring that JavaScript’s single thread can handle UI rendering, event handling, and other tasks efficiently. The process involves several key components:

1. Call Stack: This is where the JavaScript engine tracks function execution. When a function is executed, it’s pushed onto the stack, and when completed, it’s popped off.
2. Event Queue: Asynchronous callbacks (like “events”, “HTTP requests”) are placed in this queue and wait for the call stack to become empty.
3. Event Loop: Its primary job is to monitor the call stack and the event queue. When the call stack is empty, the event loop takes the first event from the queue and pushes it to the call stack to be executed.
4. Web APIs: These are provided by the browser (like “DOM”, “AJAX”, “setTimeout”) and can handle certain tasks in the background (outside of the JavaScript engine). Once these tasks are complete, the callbacks associated with them are moved to the event queue, waiting for the event loop to push them onto the call stack.

Q14. What is the difference between == and === in JavaScript?

In JavaScript, “==” and “===” are both comparison operators, but they differ in how they compare values:

• “==” is the equality operator, also known as the loose equality operator. It compares two values for equality after performing type coercion if the types are not the same. This means it attempts to convert and compare operands of different types to the same type before comparing them. For example, 0 == ‘0’ evaluates to “true” because the string ‘0‘ is coerced to the number ‘0‘ before the comparison.
• “===” is the strict equality operator. It compares both the value and the type of two operands without performing type coercion. If the types of the two operands are different, the comparison immediately returns “false”. For instance, 0 === ‘0’ evaluates to “false” because the operands are of different types: one is a number, and the other is a string

Q15. What are arrow functions in JavaScript, and how do they differ from traditional functions?

Arrow functions, introduced in ES6, are a more concise way to write functions in JavaScript. They differ from traditional function expressions in several ways:

1. Syntax: Arrow functions have a shorter syntax. They eliminate the need for the “function” keyword, using an arrow (“=>”) instead to separate the parameters from the function’s body. For example, const add = (a, b) => a + b; is an arrow function that adds two numbers.
2. “this” binding: Unlike traditional functions, arrow functions do not have their own “this” context. Instead, “this” is lexically bound, meaning it uses “this” from the surrounding code where the arrow function is defined. This is particularly useful in callbacks and event handlers, where traditional functions might require binding or using a variable to reference “this”.
3. “arguments” object: Arrow functions do not have their own “arguments” object. If we need to access the arguments in an arrow function, we can use rest parameters instead.
4. Constructor: Arrow functions cannot be used as constructors. They cannot be called with “new” because they do not have a [[Construct]] method.
5. Implicit return: If the arrow function’s body has only one statement, it can implicitly return the value of that statement without using the “return” keyword provided, the statement is not wrapped in curly braces.

Q16. Can you explain what JSON is and its common use cases?

JSON, which stands for JavaScript Object Notation, is a lightweight data-interchange format that is easy for humans to read and write and easy for machines to parse and generate.

JSON is based on a subset of the JavaScript Programming Language, Standard ECMA-262 3rd Edition – December 1999.

The JSON format is completely language-independent but uses conventions familiar to programmers of the C-family of languages, including C, C++, C#, Java, JavaScript, Perl, Python, and many others.

The common use cases of JSON include:

1. Data exchange between a server and a web application: JSON is widely used in web development as a format for sending and receiving data between a client and a server. It’s particularly popular in AJAX (Asynchronous JavaScript and XML) programming.
2. Configuration files: Many modern development tools and frameworks use JSON files for configuration. Examples include “package.json” in Node.js projects, which manages project dependencies and scripts.
3. Web APIs: Many web services provide APIs that return data in JSON format, making it easy for developers to integrate external services into their applications.
4. Storing data: Due to its easy-to-use structure, JSON is also used for storing data, particularly in NoSQL databases like MongoDB, which stores data in a binary version of JSON called BSON.

Q17. How can you prevent default browser behavior in JavaScript?

To prevent default browser behavior in JavaScript, we can use the “event.preventDefault()” method within an event handler function. This method stops the browser from executing the default action associated with the event.

For example, to prevent a form from submitting when the submit button is clicked or to stop a link from navigating to a URL when it is clicked, we can attach an event listener to the form or link and call “preventDefault()” on the event object passed to the handler.

This technique is commonly used to create custom behavior for form submissions, links, or keyboard actions, allowing developers to fully control the user experience with JavaScript.

Q18. What are template literals in JavaScript?

Template literals are a feature in JavaScript that provides a more convenient syntax for creating strings.

They allow for the inclusion of embedded expressions, which can be variables, expressions, or even function calls. Template literals are enclosed by backticks (`) instead of single or double quotes, which allows for multi-line strings without the need for concatenation.

Additionally, they support string interpolation, where placeholders are denoted by “${expression}”, making it easier to build dynamic strings

Q19. How do you ensure your JavaScript code runs after the HTML document is loaded?

To ensure JavaScript code runs after the HTML document is loaded, we can use the “DOMContentLoaded” event. This event fires when the initial HTML document has been completely loaded and parsed without waiting for stylesheets, images, and subframes to finish loading. We attach an event listener to the document object for this event and place our code inside the callback function.

This approach is widely used for initializing JavaScript functionality that depends on the DOM being fully parsed and available, ensuring that elements can be safely manipulated and event listeners added without errors.

Q20. What are JavaScript modules, and how do you use them?

JavaScript modules are a way to split the JavaScript code into separate files, making it more maintainable, reusable, and organized. Each module is a piece of code that is executed once it is loaded. In modern JavaScript, modules are defined using the “export” and “import” statements.

• Export: We can export functions, objects, or primitive values from a module so they can be used by other parts of our application. To export, we use the “export” keyword before the declaration, like so: export const name = ‘value’; or export function myFunction() {}.
• Import: To use the exported members in another module, we use the “import” statement. For example, import { name } from ‘./module.js’; imports the “name” export from “module.js”.

Modules help encapsulate functionality, avoiding namespace pollution and dependency management. They are supported natively in modern browsers through the <script type =”module”> tag and in Node.js with the “.mjs” extension or by setting “type”: “module” in the “package.json”.

This modular approach facilitates easier debugging, better code organization, and the sharing of code across different parts of our application.

Q21. What is a memory leak in JavaScript, and how can you prevent it?

A memory leak in JavaScript occurs when a program continues to hold onto the memory that is no longer needed, leading to decreased performance and, in severe cases, application crashes. This can happen when objects, listeners, or data structures are created but not properly released after their use, preventing the garbage collector from reclaiming the memory.

To prevent memory leaks, we can:

1. Properly manage event listeners: Remove event listeners when they are no longer needed using “removeEventListener”.
2. Use weak references: With “WeakMap” and “WeakSet”, references to objects are not strong enough to prevent garbage collection, helping manage memory more effectively.
3. Avoid global variables: Minimize the use of global variables or objects, as they stay in memory for the life of the application.
4. Be cautious with closures: Ensure that closures do not inadvertently keep references to large objects or the DOM elements that are no longer needed.
5. Profile memory usage: Use browser development tools to monitor memory usage and identify leaks by observing the growth of memory consumption over time

Q22. How do you perform deep cloning of an object in JavaScript?

To perform a deep clone of an object in JavaScript, ensuring that all nested objects are also cloned instead of just copied by reference, we can use several methods:

1. Using “JSON.parse()” and “JSON.stringify()”: This is a simple but effective technique for objects that contain only JSON-serializable values. We serialize the object to a JSON string and then parse that string back to a new object

However, this method does not work with functions, “Date” objects, “RegExp” objects, “Map”, “Set”, and other non-serializable values.

2. Using the “structuredClone()” function: Available in modern environments, it allows deep cloning of various types, including those not supported by JSON methods, like “Date”, “Map”, “Set”, etc.
3. Manual Recursive Cloning: For more control or specific cloning needs, we can write a recursive function to manually clone each property and check for nested objects, handling each type as necessary.

The method chosen depends on the specific requirements and the environment where the code runs, considering factors like the types of values in the object and performance implications.

Q23. Can you explain the difference between null and undefined in JavaScript?

In JavaScript, both “null” and “undefined” represent the absence of value, but in different contexts:

• “undefined” is the default value of a variable that has been declared but not assigned a value. It’s also the value returned by a function that doesn’t explicitly return anything. “undefined” indicates that the variable’s value is not defined.
• “null” is an assignment value that can be assigned to a variable as a representation of no value. Unlike “undefined”, “null” is used by programmers to intentionally denote that a variable is empty or to represent the end of a chain of object references.

Therefore, “undefined” signifies a variable has been declared but not initialized, whereas “null” is used by developers to explicitly indicate the absence of a value or a non-existent object reference.”

Q24. What is the importance of using strict mode in JavaScript?

Strict mode in JavaScript is a way to opt for a restricted variant of JavaScript, thereby implicitly making it easier to write ‘secure’ JavaScript. By declaring ‘use strict’; at the beginning of a script or function, developers can opt into this stricter context. Strict mode introduces several changes to normal JavaScript semantics:

1. Eliminates some JavaScript silent errors by changing them to throw errors, thereby allowing developers to catch and fix bugs more easily.
2. Prevents or throws errors for unsafe actions, such as gaining access to the global object.
3. Disallows variable and function declarations from being hoisted, which can lead to more predictable behavior of the code.
4. Restricts certain syntax, making it easier to avoid common coding pitfalls, such as using reserved keywords for variable names.

Using strict mode can help in the early detection of potential bugs and enforce a cleaner, more consistent coding style. It’s especially important in larger codebases where such errors can be difficult to track down.

Q25. How do you debug JavaScript code?

To debug JavaScript code, we use a combination of techniques and tools to identify and fix issues:

1. Browser Developer Tools: Modern browsers come with built-in developer tools that provide a powerful environment for debugging. I frequently use the Console to log output and the Debugger to set breakpoints, step through code, inspect variables, and watch expressions.
2. “console.log()” Statements: Inserting “console.log()” statements at various points in the code helps to quickly check the values of variables and the flow of execution.
3. Linting Tools: Tools like “ESLint” help identify syntax errors and potential bugs and enforce a consistent coding style before runtime.
4. Testing Frameworks: Writing tests using frameworks like Jest or Mocha can help catch bugs early and ensure that code changes do not introduce new errors.
5. Source Maps: When working with transpiled languages or minified code, source maps are invaluable for mapping the code running in the browser back to the original source code, making debugging much easier.

Each method has its strengths, and often, we can use a combination of these techniques depending on the specific issue and context of the project.

Q26. What do you mean by transpiled languages?

“Transpiled languages” are programming languages that are converted (transpiled) into another target language that can be executed by the environment in which the application runs.

In the context of web development, this typically means languages that are not directly understood by browsers (like TypeScript, CoffeeScript, or newer versions of JavaScript such as ES6 and beyond) are transpiled into a version of JavaScript that is widely supported by current browsers.

The transpilation process allows developers to use advanced features, improved syntax, or specific language benefits not available or supported in the target environment.

For instance, TypeScript introduces static typing to JavaScript, enhancing code robustness and developer productivity. However, since browsers do not understand TypeScript natively, it must be transpiled into plain JavaScript before it can be executed in a browser.

Transpilers, such as Babel for JavaScript, are tools that perform this conversion, allowing developers to write code in a source language and then automatically compile it into a target language that the execution environment can process.

This process is crucial for ensuring compatibility and taking advantage of modern programming language features while maintaining support for older environments.

Q27. What are the new features introduced in ES6 (ECMAScript 2015) and later versions?

ES6, or ECMAScript 2015, introduced several significant features to JavaScript, enhancing the language’s capabilities and syntax. Key features include:

1. Let and Const: For block-scoped variable declarations, allowing for better scope control than the function-scoped “var”.
2. Arrow Functions: Offering a more concise syntax for writing functions and lexically binding the “this” value.
3. Template Literals: For easier string interpolation and multi-line strings.
4. Default Parameters: Allowing functions to have default values if no argument is provided.
5. De-structuring Assignment: Enabling unpacking of values from arrays or properties from objects into distinct variables.
6. Rest and Spread Operators: For working with arrays and objects more flexibly.
7. Promises: Providing a more robust way to handle asynchronous operations.
8. Modules: Introducing native module support via “import” and “export” syntax.
9. Classes: Offering a clearer, more concise syntax for creating objects and dealing with inheritance.

Later versions have continued to build on ES6, introducing features like “async/await” for asynchronous programming, “Object.entries()” and “Object.values()” in ES2017, and “BigInt” for large integer values in ES2020, among others.

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